Turbine blades are the primary elements of wind turbines for converting wind energy into electrical energy. The working principle of the blades resembles that of an airplane wing. The blades have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
The lift force is generated when the flow from the leading edge to the trailing edge creates a pressure difference between the top and bottom surfaces of the blade. Ideally, the flow is attached to the top surface from the leading edge to the trailing edge. However, when the angle of attack of the flow exceeds a certain critical angle, the flow does not reach the trailing edge, but leaves the surface at a flow separation line. Beyond this line, the flow direction is generally reversed, i.e. it flows from the trailing edge backward to the separation line. A blade section extracts much less energy from the flow when it separates.
Flow separation depends on a number of factors, such as incoming air flow characteristics (e.g. Reynolds number, wind speed, in-flow atmospheric turbulence) and characteristics of the blade (e.g. airfoil sections, blade chord and thickness, twist distribution, pitch angle, etc). The detached-flow region also leads to an increase in drag force, mainly due to a pressure difference between the upstream attached-flow region and the downstream detached-flow region. Flow separation tends to be more prevalent nearer the blade root due to the relatively great angle of attack of the blade flow surfaces in this region as compared to the blade tip.
Hence, in order to increase the energy conversion efficiency during normal operation of the wind turbine, it is desired to increase the lift force of the blades while decreasing the drag force. To this purpose, it is advantageous to increase the attached-flow region and to reduce the detached-flow region by moving flow separation nearer the trailing edge of the blade. This is particularly desirable nearer to the blade root in order to increase the overall lift generated by the blade.
It is know in the art to change the aerodynamic characteristics of wind turbine blades by adding dimples, protrusions, or other structures on the surface of the blade. These structures are often referred to as “vortex generators” and serve to create local regions of turbulent airflow over the surface of the blade as a means to prolong flow separation and thus optimize aerodynamic airflow around the blade contour. Conventional vortex generators are typically sheet metal and defined as “fins” or shaped structures on the suction side of the turbine blade. Conventional vortex generators are not, however, without drawbacks. Vortex generators create drag and tend to be noisy.
Accordingly, the industry would benefit from an aerodynamic vortex generator design that creates less resistance and noise and is thus particularly useful nearer to the blade root for increasing the lift generated by this region of the blade.